APOBEC3 STRUCTURAL STUDIES ABSTRACT The discovery of the antiviral APOBEC3 enzymes is regarded as one of the most therapeutically promising breakthroughs in HIV/AIDS molecular virology. Several APOBEC3s have the potential to restrict HIV replication by incorporating into assembling viral particles, physically interfering with the progression of reverse transcription, and deaminating viral cDNA cytosines to uracils. The latter antiviral activity is the defining hallmark of APOBEC3-mediated restriction, explaining the genomic strand G-to-A mutations that are frequently observed in patient-derived viral sequences. However, these potent antiviral activities are counteracted by the HIV virion infectivity factor (Vif), which heterodimerizes with CBF-beta in order to form an E3 ubiquitin ligase complex that degrades APOBEC3 enzymes. Here, we will address two persisting problems in this field. First, we will determine x-ray structures of APOBEC3-bound Vif/CBF-beta ubiquitin ligase complexes. These structures will provide insights into the APOBEC3-Vif binding mechanism, conformational changes, and the overall organization of these host-pathogen complexes. Second, we will elucidate X-ray structures of APOBEC3/single-stranded DNA complexes. These structures will provide insights into the mechanisms of enzymatic catalysis, DNA substrate recognition, and coupling between DNA-binding and the release of catalytic activity. We anticipate that structural information from both APOBEC3/Vif/CBF-beta and APOBEC3/single-stranded DNA macromolecular complexes will be important in the longer-term as the field continues to move toward drugging these interactions and enabling potent HIV-1 restriction through natural innate immunity.